An antenna sending and receiving signals at multiple frequency bands is known. For example, a known multi-frequency antenna device sending and receiving signals at multiple frequency bands is disclosed in JP2008-306436A (i.e., hereinafter referred to as Patent reference 1). The known multi-frequency antenna device disclosed in the Patent reference 1 is provided with multiple numbers of monopole antennas. The monopole antennas are provided on one surface of a circuit board via a strip conductor, and a ground plate is provided on the other surface of the circuit board. The strip conductor, the circuit board, and the ground plate forms a micro-strip line, and each of the monopole antennas are fed with electricity via the micro-strip line.
According to the known multi-frequency antenna device disclosed in the Patent reference 1, the strip conductor is formed in a meander line so that an entire length of the strip conductor is assumed to be approximately a half length (½) of the wavelength of resonant frequency. An impedance converter is provided between the strip conductor and a coaxial cable connected to the strip conductor to match the impedance. Settings for the impedance converter are complex, and design and development of the impedance converter requires time and effort. Further, because forming the strip conductor in meander line geometry increases the dimensional area of the strip conductor, the dimension of the antenna device is increased.
A need thus exists for a multi-band monopole antenna which is not susceptible to the drawback mentioned above.